1. Field of the Invention
The present invention relates to an apparatus and method for image conversion and in particular relates to an apparatus and method for color image conversion for changing the number of colors used in a multi-color document and image conversion for changing the number of gradations of a monochrome document or converting the monochrome document into a multi-color document.
2. Discussion of the Related Art
By the spread of a color printer or a full-color copying machine having an edit function or the like, a circulation rate of the color document has been recently raised. However, a monochrome printer and a monochrome copying machine are still mainstream, and accordingly, there are many cases of making a copy of a color document by the monochrome copying machine or printing a color image edited with a color CRT by the monochrome printer. In outputting by the ordinary monochrome printer or monochrome copying machine, only value information of color information of input document is used. Therefore, if two regions having almost no difference in value, though differ in hue or chroma, are adjacent to each other, these regions cannot be distinguished in the output document. The applicant of the present invention has suggested an apparatus for automatic conversion of color region of a document into a hatched region disclosed in Japanese patent application unexamined publication No. Hei. 6-208357 (1994) and an apparatus for automatic conversion of a color character in a document into a monochrome character of a larger size disclosed in Japanese patent application unexamined publication No. Hei. 7-121733 (1995). These inventions enable the output of a document converted into a monochrome one in which adjacent regions can be distinguished.
Since the quality of color arrangement or likes or dislikes of a receiver is taken into consideration, it is difficult for ordinary users to convert a monochrome document into a color document. Therefore, documents with bad color arrangement are often seen, and besides, there are many cases that the receiver of a document dislikes its color arrangement. As a result, sometimes operation of restoring the color document to monochrome status is required for re-edit.
In the above-described inventions disclosed in Japanese Patent Application Unexamined Publications Nos. Hei 6-208357 and Hei. 7-121733, only the output of monochrome-completed document is supposed, and accordingly, if an obtained document is desired to be re-colored by a color copying machine or the like, it is necessary to remove hatching, or carry out contraction or enlargement of character size. Even in the case of re-color of a document having only monochrome density information such as obtained by copying a color document by the ordinary monochrome copying machine, the density information must be removed. The information regarding hatching, character size and density to be removed has a purpose of distinguishing regions; therefore, removal of information and coloring should be simultaneously executed on every region, thus requiring much labor for re-coloring.
Limitation is imposed on the output device in the case of not only a monochrome output device but also a +1 color or +N color output device. In such a case, a third color cannot be effectively used because it is supposed in the above-described inventions of Japanese Patent Application Unexamined Publications Nos. Hei. 6-208357 and Hei. 7-121733, that the output image is printed in two colors. There is a technique generally called quantizing of an image, representative of which is an invention disclosed by Japanese Patent Application Unexamined Publication No. Sho 62-105276 (1987). However, in this technique, similar colors in a image having successive color distribution such as a photograph are integrated into one color; consequently, in the case of conversion into a small number of colors, for example, three colors, high risk of wrongly integrating colors which should be clearly distinguished into the same color occurs.
To carry out re-coloring, solid color should be used or only a frame should be left instead of using hatching, a dither pattern, changing of size or displaying gradations. The solid color can be converted into another color with ease by, for example, color conversion function of a color copying machine. In the case where only the frame is left in the document, coloring can be easily realized by using, for example, coloring function of the color copying machine.
From a viewpoint of easy coloring, a general method of edge extraction can be considered. However, portions which are better to be left as they were, such as characters, are also changed into outlines, thus deteriorating appearance of the document.